TBCs are structural coatings applied to components, which are subjected to high temperatures, often greater than 1000 C, and thus would be prone inter alia to oxidation and corrosion processes. Typical applications are in the aviation and power generation industries, particularly in the coating of turbine components, such as turbine blades, liners, tiles, etc.
Existing TBCs are predominantly formed from yttria stabilized zirconia (YSZ), though other ceramic materials, such as pyrochlores, are now being considered.
Turbine components which are exposed to elevated temperatures are usually made of temperature-resistant nickel or cobalt based superalloy materials. These materials are usually coated with a bondcoat, typically a platinum alumide, platinum diffused or MCrAlY alloy, which provides for growth of a protective alumina layer, which is then covered by a ceramic TBC, which typically is made of stabilized zirconia, for example, 8 wt % yttria stabilized zirconia.
Other materials have emerged for TBCs, such as ceria stabilised zirconia, scandium stabilised zirconia or other rare earth oxide stabilised zirconias.
TBCs function by conferring reduced thermal conductivity, which, with cooling of the underlying component, reduces the thermal impact on the component material. TBCs typically provide for operation at surface temperatures of 1200 C, which would otherwise lead to premature failure.
TBCs are usually deposited using either a physical evaporation method, e.g. EBPVD, or an air plasma spray (APS) method. Physical evaporation methods provide for the formation of columnar structures on the bondcoat, which are extremely strain tolerant. Air plasma spraying is more cost effective and provides for very porous coating structures, which reduce the thermal conductivity of the TBC.
The dominant failure mechanism of TBCs is delamination, owing to oxidation of the bondcoat. Under thermal load, the bondcoat oxidizes, and an oxide layer grows at the bondcoat-TBC interface. The rate of oxidation is dependent upon temperature and increases with increasing temperature. The growth of this oxide layer results in additional stresses, which cause the formation of cracks, which grow parallel to the bondcoat-TBC interface, leading to spallation and finally to delamination of the TBC.
Failure of TBCs is also promoted as a result of sintering of the surface of the TBC. Sintering will occur when the material of the TBC is exposed to temperatures above predeterminable temperatures. For example, sintering of YSZ occurs when exposed to temperatures above 1200 C. These temperatures are predicted for future turbines, which aim for higher efficiencies by running at higher temperatures, and are already experienced in current engines, for example, in the combustion chambers.
Sintering of the surface of the TBC causes the formation of a surface layer which is much denser than the underlying bulk of the TBC, which does not experience the high surface temperatures. This structure of dense, sintered material overlying an as-deposited sub-surface material results in a TBC with reduced strain tolerance. Under a thermal gradient and cycling conditions, this lack of strain tolerance will cause cracking, which will start at material interfaces and grains, such as at pores. Once these cracks have formed, the cracks will grow and penetrate the surface area until parts of the surface of the TBC spallate. This spallation results in thinning of the TBC, which in turn provides for reduced thermal insulation, leading to an increase in the temperature of the bondcoat and accelerated growth of the oxide layer at the bondcoat-TBC interface, leading eventually to delamination of the entire coating.
Further, failure of TBCs can be promoted as a result of volume changes which occur as a result of phase transformations. For example, in YSZ, high-temperature cycling can cause the YSZ to form small amounts of the monoclinic phase, which has a different volume than the original t′ or cubic phase. The monoclinic phase forms within the t′ or cubic phase, leading to increased stresses, and eventually cracking as described above.
In general, sintering or volume changes in TBCs, which occur as a result of high-temperature exposure, in combination with frequent cycling, cause damage to the TBC, leading to crack formation. Thus, the consideration to date in developing TBCs has been in developing materials which have a reduced tendency to sinter at high temperatures and a reduced tendency to form additional phases, typically through the use of phase stabilisers.